In recent years, a light-emitting diode (LED) has improved its efficiency and has been increasingly applied to lighting apparatuses. Most conventional lighting apparatuses combine an InGaN-based blue LED and a fluorescent agent. Unfortunately, the use of the fluorescent agent inevitably causes stokes loss in principle, and not all lights received by the fluorescent agent can be converted into other wavelengths. In particular, this problem is prominent in regions of yellow and red having relatively longer wavelengths than that of blue.
To solve this problem, a technique combining a yellow or red LED with a blue LED has been recently adopted. In this regard, a bulb type lighting apparatus, in which LEDs are arranged on a board to form a filament for emitting light, has been widely spread rather than a COB (chip on board) type, in which light is extracted to one surface. LED devices used in the apparatus of this type need to extract light over the entire filament surface. Thus, a device that extracts light to one side is not suitable, and a device having light distribution to extract light in every direction of a chip is ideal.
For the InGaN-based LED, which is the blue LED, a sapphire substrate is generally used. The sapphire substrate is transparent to emission wavelength, and thus is ideal for the above-described lighting apparatus. For the yellow or red LED, however, GaAs or Ge, which can absorb light with emission wavelength, is used as a starting substrate, and is unsuitable for the above-described use.
To solve this problem, there have been disclosed a method in which a transparent substrate is bonded to a light-emitting portion as described in Patent Document 1; and a technique in which a window layer is grown to have a sufficient thickness to be used as a support substrate, and a starting substrate that is a light-absorbing substrate is removed to provide an LED as described in Patent Document 2.
The method disclosed in Patent Document 1 requires bonding a transparent substrate thicker than necessary, and grinding the substrate to a predetermined thickness after bonding, which can increase the cost. Moreover, the substrate used for bonding usually has a thickness of 200 μm or more. Considering light distribution characteristics and assembly properties with other devices, the thickness required for an LED device is approximately 100 μm at most, and thus the substrate requires thinning to this degree of thickness. The thinning processing increases the number of processes and a risk of cracking the wafer, causing an increase in cost and a reduction in yield.
On the other hand, the method disclosed in Patent Document 2, which utilizes, as a support substrate, a window layer grown by crystal growth to have a sufficient thickness for the support substrate, includes growing the window layer to a desired thickness, and does not require thinning processing and substrate joining/bonding processes. Thus, this method enables low-cost formation and is excellent.
The light-emitting device having a transparent support substrate as described above generally employs a technique for preventing multiple reflection within the light-emitting device and inhibiting light absorption in order to enhance luminous efficiency. Patent Document 3 proposes a method for roughening the surfaces of a window layer-cum-current diffusion layer and a window layer-cum-support substrate, but not roughening the surface of a light-emitting portion in a structure where the light-emitting portion is sandwiched between the thick window layer-cum-current diffusion layer and the thick window layer-cum-support substrate. However, this method requires forming a deep trench that pierces through the window layer-cum-current diffusion layer, which increases the cost, and makes a vertical distance between upper and lower electrode portions large, which causes a difficulty in wire bonding. In the application to a flip chip type, a thick insulator film and a very long metal via must be formed, which causes the increase in cost. It is thus desirable for the window layer-cum-current diffusion layer used as the upper electrode portion to be thin.
Patent Documents 4 and 5 disclose techniques in which the window layer-cum-current diffusion layer is thin, the vertical distance between upper and lower electrode portions is short, and a light-extracting portion or a light-reflecting portion has a roughened surface. In Patent Document 4, the roughened surface is formed on an n-type semiconductor layer surface at the opposite side of a light-extracting surface. However, this disclosure relates to the flip chip type, and intends to efficiently reflect light from an electrode side to a window layer side. Moreover, this document discloses a difficulty in forming roughened surfaces on both the window layer-cum-support substrate and the light-emitting portion.
Patent Document 5 discloses a technique for roughening the surface of an AlGaInP-based clad layer. According to the technique disclosed in Patent Document 5, a clad layer is made of (Al0.5Ga0.5)0.5In0.5P, and a window layer made of (AlxGa1-x)0.5In0.5P (0.5<x) is formed on an upper part of the clad layer. That is, the disclosed technique indicates that the layer to be roughened is made of a material having a higher Al content than that of the clad layer.